6.15.2 Match Structures

A match structure is the object returned by string-match and regexp-exec. It describes which portion of a string, if any, matched the given regular expression. Match structures include: a reference to the string that was checked for matches; the starting and ending positions of the regexp match; and, if the regexp included any parenthesized subexpressions, the starting and ending positions of each submatch.

In each of the regexp match functions described below, the match argument must be a match structure returned by a previous call to string-match or regexp-exec. Most of these functions return some information about the original target string that was matched against a regular expression; we will call that string target for easy reference.

Scheme Procedure: regexp-match? obj

Return #t if obj is a match structure returned by a previous call to regexp-exec, or #f otherwise.

Scheme Procedure: match:substring match [n]

Return the portion of target matched by subexpression number n. Submatch 0 (the default) represents the entire regexp match. If the regular expression as a whole matched, but the subexpression number n did not match, return #f.

(define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
(match:substring s)
⇒ "2002"

;; match starting at offset 6 in the string
(match:substring
  (string-match "[0-9][0-9][0-9][0-9]" "blah987654" 6))
⇒ "7654"

Scheme Procedure: match:start match [n]

Return the starting position of submatch number n.

In the following example, the result is 4, since the match starts at character index 4:

(define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
(match:start s)
⇒ 4

Scheme Procedure: match:end match [n]

Return the ending position of submatch number n.

In the following example, the result is 8, since the match runs between characters 4 and 8 (i.e. the “2002”).

(define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
(match:end s)
⇒ 8

Scheme Procedure: match:prefix match

Return the unmatched portion of target preceding the regexp match.

(define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
(match:prefix s)
⇒ "blah"

Scheme Procedure: match:suffix match

Return the unmatched portion of target following the regexp match.

(define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
(match:suffix s)
⇒ "foo"

Scheme Procedure: match:count match

Return the number of parenthesized subexpressions from match. Note that the entire regular expression match itself counts as a subexpression, and failed submatches are included in the count.

Scheme Procedure: match:string match

Return the original target string.

(define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
(match:string s)
⇒ "blah2002foo"